SceneKit shadow on a transparent SCNFloor()

Question

I have a floor node, on which I need to cast shadow from directional light. This node needs to be transparent (used in AR environment)

Answer 1

It's been awhile since this was posted but maybe someone will find this alternative solution useful. I encountered a similar situation and what I ended up doing was rendering using multiple passes via SCNTechnique. First I rendered a floor with a solid white diffuse and then I rendered the rest of the scene without the floor. To do this I set the categoryBitMask of my SCNFloor to 3 and left the others with the default value of 1.

Next I created my SCNTechnique with this definition which renders the floor and the rest of the scene into separate buffers and then combines them together into the final scene:

self.sceneView.technique = SCNTechnique(dictionary: [
  "passes" : [
    "store-floor": [
      "draw" : "DRAW_NODE",
      "node" : "floor-node",
      "inputs" : [],
      "outputs" : [ "color" : "color_floor" ]
    ],
    "store-scene": [
      "draw" : "DRAW_SCENE",
      "excludeCategoryMask" : 2,
      "inputs" : [],
      "outputs" : [ "color" : "color_scene" ]
    ],
    "recall-scene": [
      "draw" : "DRAW_QUAD",
      "metalVertexShader" : "vertex_tecnique_basic",
      "metalFragmentShader" : "fragment_tecnique_merge",
      "inputs" : [ "alphaTex" : "color_floor", "sceneTex" : "color_scene" ],
      "outputs" : [ "color" : "COLOR" ]
    ]
  ],
  "symbols" : [
    "vertexSymbol" : [ "semantic" : "vertex" ]
  ],
  "targets" : [
    "color_floor" : [ "type" : "color" ],
    "color_scene" : [ "type" : "color" ],
  ],
  "sequence" : [
    "store-floor",
    "store-scene",
    "recall-scene"
  ]
])

Next the Metal share code that takes those two buffers and combines them together where the alpha value ranges from 0 for white to 1 for black.

using namespace metal;
#include 

struct TechniqueVertexIn
{
    float4 position [[attribute(SCNVertexSemanticPosition)]];
};

struct TechniqueVertexOut
{
  float4 framePos [[position]];
  float2 centeredLoc;
};

constexpr sampler s = sampler(coord::normalized, address::repeat, filter::linear);

vertex TechniqueVertexOut vertex_tecnique_basic(
  TechniqueVertexIn     in [[stage_in]],
  constant SCNSceneBuffer&  scnFrame [[buffer(0)]])
{
  TechniqueVertexOut vert;

  vert.framePos = float4(in.position.x, in.position.y, 0.0, 1.0);
  vert.centeredLoc = float2((in.position.x + 1.0) * 0.5 , (in.position.y + 1.0) * -0.5);

  return vert;
}

fragment half4 fragment_tecnique_merge(
  TechniqueVertexOut  vert [[stage_in]],
  texture2d  alphaTex [[texture(0)]],
  texture2d  sceneTex [[texture(1)]])
{
    float4 alphaColor = alphaTex.sample(s, vert.centeredLoc);
    float4 sceneColor = sceneTex.sample(s, vert.centeredLoc);
  float alpha     = 1.0 - max(max(alphaColor.r, alphaColor.g), alphaColor.b); // since floor should be white, could just pick a chan

  alpha *= alphaColor.a;
  alpha = max(sceneColor.a, alpha);

  return half4(half3(sceneColor.rgb * alpha), alpha);
}

Lastly here's an example of what that ends up looking like with all the pieces put together.